The basic assumption in using steel reinforcement in concrete is that their thermal expansion coefficient are almost of same order and therefore with increase or decrease of temperature the bond between steel and concrete will remain intact..

However, still the question is important from the point of view that if the temperature near the bar is high due to any reason and on the surface concrete is cooler then what will happen which is normally the case during surface intermittent curing?

Probably a mathematical model has to be developed for calculation of increase in area of bar with increase in temperature to exactly know how much area increases and corresponding resulting pressure around the bar?

Apart from response received, one can go with microstructural examination (Petrographic Examination) with required degree of magnification for RCC specimen by using any appropriate methods like SEM.